Private water wells in Arizona are among more than 20% of private domestic wells sampled nationwide that contain at least one contaminant at levels of potential health concern, according to a study by the U.S. Geological Survey (USGS).
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The contaminants most frequently measured at concentrations of potential health concern were inorganic contaminants, including radon and arsenic, which are mostly derived from the natural geologic materials that make up the aquifers from which well water is drawn. Complete findings are available online.
[this is taken in large part from the USGS press release]